The US sprayed over 20 million gallons of herbicides, including 13 million gallons of Agent Orange, onto the thick forests of Southeast Asia over the course of the Vietnam War, hoping to strip the Viet Cong of food and cover. However, the powerful defoliant left a truly horrendous legacy. Thousands of Vietnamese and US troops were left with liver diseases, skin problems, and severe health complications. Worse still, thousands of civilians were affected by abnormally high incidences of miscarriage, stillbirths, and severe congenital malformations.

A new study has shown how a toxic byproduct of Agent Orange, dioxin TCDD, continues to lurk in the environment in Vietnam and the human food supply 50 years on. Reporting in the Open Journal of Soil Science, scientists detail the lengthy chain of events that has allowed dioxin TCDD to persist in many corners of the Vietnamese ecosystem, from the soil and water system to the land’s crops and fowl.

“The pathway begins with the US military spraying in the 1960s, absorption by tree and shrub leaves, leaf drop to the soil surface (along with some direct contact of the spray with the soil), then attachment of the dioxin TCDD to soil organic matter and clay particles of the soil," co-author Lois Wright Morton of Iowa State University said in a statement.

A middle-aged man who experienced significant levels of Agent Orange exposure when he was in the womb. Emilio Labrador/Flickr CC BY 2.0

Dioxin TCDD then clings to sediment particles and makes its way into wetlands, marshes, rivers, lakes, and ponds. Here, it’s eaten by bottom-feeding fish and shrimp, accumulating in their fatty tissue and then in the bellies of their predators, including many fish that are widely eaten in Vietnam and other countries in Southeast Asia.

To reach these findings, the researchers buried their heads in secondary sources that outlined soil, sediment, and water samples from numerous locations around US air bases and known hotspots of Agent Orange activity. They did not look to see how the legacy of dioxin TCDD could be affecting the health of people or wildlife in the area, instead focusing on the persistence of dioxin TCDD and its potential environmental effects.

However, they note that dioxin TCDD still remains at dangerous levels near numerous cities and towns where millions of people live. While the levels are not close to those seen during the 1960s and 1970s, the researchers recommend the incineration of contaminated soils and sediments around the known hotspots – an expensive, but perhaps necessary means to end the legacy of Agent Orange.

"The worst dioxin-contaminated site in Vietnam is Bien Hoa airbase, which is 30 miles north of Ho Chi Minh City," added co-author Ken Olson, professor emeritus in the Department of Natural Resources and Environmental Sciences at the University of Illinois.

"While incineration is the most expensive technology currently available, it would eliminate dioxin rather than temporarily store it in a landfill, and incineration would not require future maintenance or treatment," the authors conclude.